1. Field of the Invention
This invention pertains generally to the field of optoelectronic data transmitter/receiver modules and in particular concerns protective sealing of optical assemblies in such modules.
2. State of the Prior Art
Optoelectronic transmitter/receiver modules are used for communicating over optical fiber links as opposed to electrical cables. Optical transmission has advantages over electrical conductors, such as relative immunity from interference, low loss transmission over long distances, and high data rate capacity, among still others. Optoelectronic devices serve to interface electronic circuits, in which the signals are generated or processed, to optical fiber cables for transmission to other electronic circuits.
Optoelectronic transmitters convert electrical signals to optical signals for transmission via optical fiber, while optoelectronic receivers perform the opposite operation of converting optical signals received via optical fiber to electrical signals. A transmitter and a receiver may be packaged together in a transceiver module. Multichannel optoelectronic modules transmit and receive signals simultaneously over multiple parallel fibers or channels, and depending on the application, four, eight or more parallel data channels may be used. For example, a four channel transceiver module requires eight optical fibers, four fibers for transmission and four fibers for reception. Flat ribbon cable is commercially available in different widths containing varying numbers of optical fibers for interconnecting multichannel modules.
Optoelectronic modules, whether transmitter, receiver or transceiver modules, are usually packaged in a module housing containing a printed circuit board on which are mounted electronic circuits and devices for electronic signal processing and an optical assembly which interfaces the electronic circuits of the module to an optical fiber link. This optical assembly normally includes a semiconductor diode array mounted in optical alignment with the fiber facets of an optical fiber array supported in a fiber block. In a transmitter module the diode array includes light emitter diodes which are typically laser diodes, although LEDs may be substituted in some applications. The laser or LED diodes convert drive current supplied by the module's electronic circuits to a light output which is carried by the optical fibers. In a receiver module the diode array includes photo detector diodes. Light signals delivered by the optical fibers illuminate the photo detector diodes which convert these light signals to electrical signals. Transceiver modules have both a laser diode array chip and a photo detector diode array chip, and each chip is aligned with a corresponding subset of the fiber facets of the fiber block. For simplicity of explanation, the following description is limited to an optoelectronic module with a single diode array, which may be either a PE array or a PD array unless otherwise stated, and for convenience may be referred to as a PD/PE array.
Each diode array, whether emitter or detector, is typically manufactured on a single semiconductor chip where the diodes are arranged on the chip as an evenly spaced linear array on a common surface or edge of the chip. Due to differences in manufacturing processes, laser diode arrays and photo detector arrays are made on separate chips. These semiconductor chips are small and delicate, and are supported by bonding, as with epoxy adhesive, to a submount of appropriate material, such as aluminum nitride or aluminum oxide. The submount also carries electrical conductors, usually applied as printed circuit traces, for interconnecting the diodes of the array to the electronic circuits of the optoelectronic module. This interconnection is made by wire bond connections of very thin wire between the diode array chip and the conductive traces on the submount. The wire bonds can be made with conventional wire bonding machines. Each wire bond is a small arc of bare wire welded at each end to the electrodes or terminals which are electrically connected by the wire bond. The arc of the wire bond rises to some height above the electrodes, the height depending on the particular wire bonding machine used.
The optical fiber array of the module includes a number of relatively short fiber lengths supported between two substrates of the fiber block. Each fiber length of the fiber array terminates in an end facet on an end face of the fiber block. Each facet is illuminated by a corresponding light emitter diode (in a transmitter) or itself illuminates a corresponding photo detector diode (in a receiver). The opposite, outer ends of the optical fiber array are optically coupled by industry standard optical connectors to an external optical fiber cable, such as a flat ribbon cable, which interconnects the optoelectronic module to another optoelectronic module.
The diode array chip with its wire bonds is mechanically delicate and susceptible to mechanical damage or breakage while being handled in the process of alignment and assembly of the module. The diode chips are also vulnerable to degradation by airborne pollutants, while the fiber facets are adversely affected by condensation of moisture.
Optoelectronic communications modules are used in a wide range of operating environments, depending on the application. In some cases the equipment containing the modules is installed in protected, air conditioned rooms, where the modules are sheltered in a controlled environment and are readily accessible for maintenance and replacement if needed. In these cases a lesser degree of hardening and reliability of the modules is needed. In other applications, such as long haul communications, the modules may be installed in equipment exposed the outdoor environment in harsh climates, and in remote or difficult to access locations such as on mountaintops or undersea cable installations. In the latter cases, a high degree of hardening and reliability of the modules is desirable due to the high cost of repairs.
For this reason, optoelectronic modules are often packaged in relatively costly, hermetically sealed module housings. Lower cost alternate solutions have been sought for protective sealing of optoelectronic modules, particularly for less critical applications. One lower cost technique has been to encapsulate the optoelectronic components in transparent resin. A coating of transparent resin is applied over the components and, after the resin sets and hardens, it provides a relatively durable, chemically and mechanically resistant encapsulating layer.
A difficulty encountered in making such resin seals is that the resins tend to be fluid and runny in their initial uncured state, and when first applied to the optoelectronic components, the epoxy resin has a tendency to run off the component before hardening. Thicker paste resins have been used in a one step encapsulation process. Most paste resins, however, have inferior transparency and light transmission characteristics and may become too hard when set, which poses a risk of damage to the diode array chip and its delicate wire bonds. In the past this problem has been addressed by a two step encapsulation process. First, a thicker epoxy formulation, which itself may be undesirable as an encapsulating epoxy, is applied to the submount around the diode array chip to form a raised containment barrier or dam for the more fluid encapsulating epoxy resin. After the barrier epoxy has set, the more fluid encapsulating epoxy is applied over the component. This is an awkward, time consuming and labor intensive procedure, and a simpler, more expedient technique is needed.